The well-known CT is advantageous in that the detection is possible with isolation from the detected wire, but it can not be used with respect to direct current, alternating current with a frequency close to direct current, and pulsating current with direct current and alternating current superposed. Then, as an alternative to the CT, other isolation type direct current sensors were proposed such as hall element type, magnetic amplifier type, magnetic multi vibrator type, magnetic flux inversion time difference type (for example, see the patent application laid open Nos. H10-10161, H10-332745), magnetic path chopping type (for example, see the patent application laid open No. 2000-55940), etc.
In the hall element type, a gap is formed in a part of the magnetic core through which the detected wire passes and is provided with a hall element such that the value of current running along the detected wire is detected from the variation of electric characteristic value of the concerned hall element. In the magnetic amplifier type and the magnetic multi vibrator type, the value of current running along the detected wire is detected using the time difference reaching the saturation of alternating flux generated by magnetically exciting a magnetic core to the vicinity of saturation flux density in advance with alternating current to magnetically polarize it with direct current. The three types mentioned above are suitable for the case in which current sensitivity per 1 turn of the detected wire is not less than several ampere, but can not detect the current of several mA.
In the magnetic flux inversion time difference type, the triangular exciting current that exceeds sustaining force is run along the magnetic core, which the detected wire passes through, and the period of time from when the flux in the magnetic core starts to run in one direction to when the direction of the flux is inverted toward the other direction is compared with the period of time from when the flux starts to run toward the other direction to when the direction of the flux is inverted toward the opposite, so the direct current of the detected wire is detected. Particularly, in the feedback type (see the above second reference), its structure is complicated and the uniformity of quality between products is lacking, because the plurality of magnetic cores are used and the plurality of coils such as the coils wound around single magnetic core, the coils wound over the plurality of magnetic cores, etc. are present. In no feedback type (see the above patent application laid open No. H10-10161), current sensitivity per 1 turn of the detected wire is about 100 mA and dynamic range is narrow.
In the magnetic path chopping type, saturating a part of the magnetic core, which is passed through by the detected wire, with the flux of which the direction is perpendicular to that of the flux generated by the detected wire and increasing the magnetic reluctance of the magnetic core are intermittently carried out in time series. Namely, in this type of which the basic principle is identical with the well-known chopper amplifier, the value of direct magnetic flux, that is, the value of direct current running along the detected wire, is detected by chopping the direct magnetic flux generated by the detected wire to alternating magnetic flux. This type is very complicated in winding of coils, shape of the magnetic core, etc. when compared with other types.
Besides, saturation type (Cramer type), zero flux type, etc. are all for detecting high current, and measurement of current in the range of several mA is difficult.
As mentioned above, in the prior art of direct current sensor, current sensitivity per 1 turn of the detected wire is bad, and a sensor that has relatively good sensitivity is very complicated in the structure.
Currently, preservation of the environment has become an issue of public concern such that direct current like solar power, fuel cell, etc. is becoming more important and the increase of demand on direct current power equipment using the secondary cell, such as electric vehicles, hybrid vehicles, etc. is anticipated. In addition, from the aspect of energy saving, current detection by magnetic field in which energy consumption is close to a perfect zero is preferred in the art using the prior shunt resistors. In direct current and high voltage circuits used in high voltage electron tubes, etc., current tends to become lower if voltage becomes higher, and it is preferred that the current in high voltage circuit is measured under isolation.
However, in the prior art, there are no current sensor and current detecting method by which the sensitivity and dynamic range corresponding to the above-mentioned concerns are achieved and the cost of manufacturing is economical such that the wide use for the general purpose is possible.